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Stable isotopes are a powerful way to describe and quantify trophic relationships in aquatic systems. Evaluating the ratios of carbon and nitrogen isotopes of consumers and organic matter sources in aquatic systems can answer key ecological questions about the flow of energy between producers and higher trophic levels. However, this method is only feasible when sources have distinct combinations of isotopic ratios. This thesis evaluates the potential for deuterium to improve food web models in aquatic systems based on the large differences in isotopic ratios between primary producers. Evaluation of hydrogen isotopic ratios from plants and algae collected from marine and freshwater settings revealed differential incorporation of deuterium into plant organic matter. Most aquatic plants were relatively depleted in deuterium, except for macrophytes such as seagrass, which were enriched similar to many types of terrestrial vegetation. This result suggests that interpretations of allochthony from deuterium signatures alone would be complicated in a system where macrophytes are abundant. Large differences were found in hydrogen isotope ratios between different species of macroalgae that cannot usually be differentiated using other isotopes. Including hydrogen isotopic ratios in a food web model along with carbon and nitrogen revealed sources of organic matter support to Mercenaria mercenaria grown in aquaculture pens in the Virginia Coast Reserve. A Bayesian model found that macroalgae was the leading source of organic matter to clam diets, and that hydrogen isotope ratios improved model performance relative to models based only on carbon and nitrogen. The ecological implications to clam feeding are discussed in the context of potential changes to primary production in the Virginia Coast Reserve.

Using Multiple Stable Isotopes Including Deuterium (82H) to Trace Organic Matter in a Complex Near-shore Lagoon